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Retirement Benefits Plans (Amounts Recognized as Components of Other Comprehensive (Loss) Income) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reclassifications to earnings of:      
Prior service benefit amortization $ (0.1) $ 0.0 $ 0.1
Retirement benefit plan funded status adjustments:      
Actuarial (losses) gains 1.6 0.7 (0.6)
Total (0.1)   0.1
Postretirement benefit plans      
Reclassifications to earnings of:      
Actuarial loss amortization 0.7 0.6 0.6
Prior service benefit amortization (0.5) (0.6) (0.7)
Retirement benefit plan funded status adjustments:      
Actuarial (losses) gains (2.0) (1.0) 0.8
Total $ (1.8) $ (1.0) $ 0.7